Health & Safety and Technical notes

The coil should be loosely wound from 1 m of polyurethane-coated copper wire (30 or 33 SWG). Strip coating from the ends to allow electrical contact via crocodile clips.

Procedure

a Set up a simple series circuit with long leads to the loosely wound coil of copper wire.

b Adjust the power supply to give a current of about 4-5 amps in the coil. Switch the circuit off, as soon as possible.

c After a minute or so, the coil will have cooled to room temperature. Switch the circuit on again. Take readings of the ammeter and voltmeter several times during the next half-minute or so. During this time the coil heats up and the current changes quite rapidly.

d Repeat the experiment with the coil of copper wire suspended in water in the container. The water should be kept very well stirred. Take care to avoid short-circuiting the coil by using a wooden lolly stick or spatula as the stirrer.

Teaching notes

1 Students record pairs of current and potential difference readings with the coil in air, and then plot the current/potential difference characteristic. This is not a straight line showing constant resistance, but rather a curve showing that the resistance of the wire increases with temperature.

2 When the experiment is repeated with the coil in a water bath so that its temperature remains constant, the characteristic graph is a straight line, showing that the resistance remains constant. Pure metals do obey Ohm's law when their temperature remains constant. Wires made from alloys such as Constantan or Eureka wire (consisting of 60% copper and 40% nickel) are designed to have a very small temperature coefficient of resistivity. Therefore, they do not need to be placed in a constant temperature bath in order to show ohmic behaviour.

3 How Science Works Extension: This experiment can be used to teach about the idea of validity of scientific results. Results may be rendered invalid if an uncontrolled factor affects the results. In this case, the temperature of the wire is a factor which affects measurements of its resistance.

Discuss how this can be taken account of. One approach is (as discussed above) to keep the wire at a constant temperature by immersing it in a water bath. An alternative would be to use the p.d./current graph above to find the wire’s resistance when no current flows through it, because then there is no heating effect. Explain that the resistance of the wire is equal to p.d. divided by current; i.e. it is the gradient of the line from the origin to the point on the graph. Place a ruler on the graph, through the origin, and passing through the highest point on the graph. The ruler has a large gradient. Move down the graph, from point to point, showing that the gradient decreases. Close to the origin, the graph is almost straight (or you can use the idea of the tangent to the graph). In this way, you can determine the resistance of the wire when it is not heated by the current.

4 This experiment can be extended to include an investigation of the effect of temperature on resistance, between 0°C and 100°C, using a water bath. If students are familiar with the experimental observation of Charles’ law, you could ask them to extrapolate their graph of resistance against temperature to find the approximate temperature at which the wire’s resistance would be zero. For a pure metal, resistance decreases approximately linearly towards a temperature close to 0 K. (The temperature coefficient of resistance of many pure metals is close to 0.004 K-1, so the resistance/temperature graph will extrapolate back to 1/0.004 = 250 K.) You could link this to the idea that the resistance of a pure metal at room temperature is dominated by the vibration of ions, and this will reduce to zero close to 0 K.